There are conditions where broodstock transport becomes inevitable. This can be because of spawning practice, sale of fish or to avoid environmental conditions. For whatever reason, the transport of broodstock will mean an impact on the fish and therefore, gamete quality.

There are direct and indirect implications on egg survival that result from the physiological response to a physical change. Minimizing the impact of these physical changes, called stressors, are what really matters to both the fish and the outcome of the spawning year.

Key to minimizing the fishes’ response to stressors is understanding some of the physiology of the response mechanism. The word ‘physiology’ is a stressor to some people, but really, it can be simply explained.

The fish responds to most stressors, be they physical (handling), environmental (low oxygen) or social (crowding) in the same manner. Sadly, fish do not have a fight mechanism in the ‘flight or fight’ response. They have limited resources at their disposal, so all their actions look stupid to us: they try and inflict as much damage to themselves by thrashing about instead of cooperating.

Obviously, there are the physical damage factors that are minimized while handling fish. Try and crowd fish in shifts, that is, shallow only a few fish at a time in a net and hold only a few fish at a time in an anesthetic bath (into which you have used and approved anesthetic). Try not to hold or transport fish in structures designed along the lines of an iron maiden. Surfaces of contact should be smooth and where possible, padded. Avoid really bright colours and direct lights. Seine and dip nets should be of close mesh, high weight and knotless.

Try and minimize the times the fish are handled. If transport is necessary, sort and transport on the same day – repeat stress is very damaging. Be aware of supplemental oxygen levels (more on this below). Finally, do not handle fish that are not out fully and do not leave fish in an anesthetic bath for extended periods.

The intense desire to escape has both immediate and delayed physiological responses. The first response is identical to what humans feel and is fueled by the same chemical: adrenalin (epinephrine). The adrenergic response is to increase metabolism by getting sugar into muscle cells and putting the nervous system on high alert. Respiration rate, heart rate and blood pressure also increase. In short, it increases metabolic rate. Read ‘panic’.

In knowing this, one can prepare by handling the fish slowly, carefully and with a minimum of agitation. It is not natural to be netted and dumped in a tote whether you are human or beast. Don’t expect the fish to like it.

The second physiological response is subtler and occurs after the stress is gone. In preparation for more stressful experiences, or in response to a prolonged stress – like transport – the steroid hormone cortisol is released. It works like this: the brain perceives the stressor; it releases a chemical messenger (ACTH) to contact the site of cortisol manufacture in cells of the kidney. They release cortisol, which affects many types of cells. These target cells then liberate glucose by affecting carbohydrate, lipid and protein metabolism. In other words energy reserves are made available for further flight.

There is a dark side to high cortisol levels. In preparation for further stress, cortisol puts other functions on hold. These include immune responses and reproduction. Consider fish that have been repeatedly handled: they often get sick and many of them can revert to silvers when they looked like they were a sure thing. Many egg projections have been trashed because fish were handled at a sensitive time or too often.

In fish that are handled repeatedly, meaning in consecutive days, cortisol response is compounded: it is more than an additive effect. It takes about 24-48 hours at 10°C for fish to recover. Spawners could take longer; nobody knows for sure.

Environmental conditions affect cortisol release as well. High oxygen levels (those above 110% saturation) cause a condition of acidosis in fish. This is in response to low ambient carbon dioxide relative to oxygen. The fish tries to maintain an internal balance of oxygen/carbon dioxide. When external oxygen is high, the balance is hard to maintain and internal levels of carbon dioxide increase. This increase produces acids that change blood pH. A change in blood pH for extended periods has a direct effect on the fish (and eggs) as well as releasing cortisol. So, more oxygen is not better, this includes baths and transport.

Other stress indicators are blood lactate and blood glucose. While these are indicators of increased metabolism, they are only part of a stress response. There is no single part of the stress response that characterizes a cataclysmic event that results in failed spawning or death of the fish. Each part of the response is a piece of an overall effect that will impact spawning performance. Some are more harmful than others and the overall effect of compounding factors is the make or break point.